The invention described herein relates to a method for manufacturing high purity cobalt for use in sputter targets and related microelectronics applications. High purity cobalt is defined as having an oxygen content of not more then 500 ppm, a Ni content not more than 200 ppm, an Fe, Al and Cr content not more than 50 ppm each, and Na and K not more than 0.5 ppm. The cobalt target exhibits a low magnetic permeability in the plane of the target and higher magnetic permeability normal to the target surface, i.e. large surface flux leakage. The invention is a method of fabrication of high purity cobalt with a grain structure which has a strong preferred crystallographic orientation in the hcp phase and little or no detectable fcc phase. This strong hcp crystallographic texture, (tilted (0002) plane), is critical to the sputtering efficiency and material utilization of the target used in the stated application.
High purity cobalt targets are used in sputtering applications to produce thin films on microelectronic devices such as microprocessors, DRAM""s, etc. DC magnetron sputtering efficiency relies heavily on the ability of a magnetic field to trap electrons liberated from the target, and direct them back to the negatively charged target where they are repelled. These electrons spiral through the plasma gas (typically argon) in the sputtering chamber towards the target. This spiraling motion increases the frequency of collisions with argon atoms, which results in more argon atoms striking the target and ultimately higher deposition rates. The sputtering rate is increased (to some asymptotic limit) by increasing the strength of the component of the magnetic field parallel to the target surface. This in turn relies heavily on the ability of the magnetic field to penetrate the target material. Cobalt, which is ferromagnetic, is very anisotropic in its magnetic properties, and can often resist field penetration. Consequently, when a magnetic substance such as conventionally processed cobalt is used as a target, the magnetic flux tends to pass through the interior of the target and only a low flux can exit into the plasma discharge space. This problem requires the use of very thin cobalt targets which as a result have relatively short service lives. Furthermore, the local cross-sectional decrease of the target during the sputtering (erosion trench) brings about an increasing magnetic flux directly over the erosion trench. This causes a higher ionization probability of the sputtering gas to occur in the region and a higher sputtering rate to occur locally, with the consequence that the erosion trench becomes very narrow, resulting in poor material utilization. The effect is compounded with the use of thin targets because of the exponential relationship between magnetic field strength and the distance from the magnet. A small amount of erosion on a thin target produces a much greater localized magnetic flux strength increase than would an equivalent amount of erosion on a thicker target, due to the fact that the thicker target surface is further from the magnetron magnet.
To combat these problems, a low magnetic permeability value in the plane of the high purity cobalt target is needed so that the magnetic flux leakage normal to the target is increased. This will allow target thickness to be increased, thereby extending target life, and improving sputtering efficiency and performance.
Cobalt has two crystalline forms-face centered cubic (fcc) and hexagonal close packed (hcp). The hcp phase is the low temperature crystal structure and exists up to 422xc2x0 C.; above this temperature it transforms into the fcc phase. A conventional high purity cobalt sputtering target hitherto used comprises a mixture of these two phases. This conventional target is manufactured by a process wherein high purity cobalt material is heated until it is melted, and then poured into a mold. At this temperature the material it is in the fcc single phase. The cast ingot is then allowed to cool immediately or after it has been subjected to hot-working treatment, so that part of the fcc single phase is transformed into a martensitic structure which includes a hcp phase. The conventional high purity cobalt thus manufactured has been used for magnetron sputtering.